Water structure in [Phe4 Val6] antamanide • 12H2O crystallized from dioxane

Crystals of [Phe⁴ Val⁶] antamanide (cyclic[ValProProPhePhe]₂) grown from dioxane/H₂O, with space group P2₁2₁2 and cell parameters a= 15.099(4), b = 22.008(5) and c = 11.024(3) Å, are almost identical to crystals grown from H₂O/acetone, the structure of which was determined a number of years ago. Perpeptide molecule there are the equivalent of 12 water molecules occupying 16 sites in both crystals; however, in the new investigation a number of water molecules present at one-half occupancy have been found in different positions than in the earlier analysis. The interpretation of the hydrogen bonding between peptide/water and between water/water is much more satisfactory. Pentagonal water assemblies are present in the solvent channel. There is a distinct indication of the occurrence of a bifurcated bond between two water molecules, as well as the presence of three-center hydrogen bonds joining three water molecules. This may be the first experimental example of a bifurcated bond between two water molecules.